Metal roof trusses may take on various shapes, but the most commonly encountered shape is defined by two upper chords joined at adjacent ends and each connected to a lower chord to create a generally triangular truss. Extending between the upper and lower chords are a plurality of web members.
The prior art teaches various cross-sectional shapes for the upper and lower chords, including generally U-shaped or C-shaped chords. Prior art chord shapes are taught at U.S. Pat. No. 5,771,653 to Dolati, which is incorporated herein in its entirety for all purposes. The prior art also teaches J-shaped and Z-shaped chord members, as well as other shapes.
The prior art also teaches various cross-sectional shapes for the web members of metal trusses, namely, C-shaped web members or rectangular tubular members. For example, U.S. Pat. No. 6,874,294 to Masterson teaches a generally C-shaped (or U-shaped) cross-section for the truss web member. Similar web member cross-sections are taught at U.S. Pat. Nos. 6,260,327 to Pellock, 6,658,809 to Collins, 5,463,837 to Dry, and 6,167,674 to Nanayakkara. Also in common use are web members of rectangular and square tubular members. The use of square and rectangular tubulars as web members is taught in, for example, U.S. Pat. Nos. 5,771,653 to Dolati, 6,088,988 to Sahramaa, 5,417,028 to Meyer, and 4,986,051 to Meyer. More exotic web member shapes, such as an H-shape and W-shape, have also been employed in the prior art. See, for example, U.S. Pat. Nos. 5,457,927 to Pellock, and 6,073,414 to Garris, et al.
The C-shaped web members have significant drawbacks. A C-shaped member is prone to flexural torsional buckling for any given gauge or thickness of metal in comparison to a tubular or S-shaped web member. This is because the center of gravity and center of shear of a C-shaped member do not coincide. Consequently, a C-shaped member requires the use of relatively thicker metal or requires lateral bracing to provide additional strength. Additionally, a C-shaped member, in torsional buckling mode, precludes the use of continuous lateral bracing along only one edge of the web member. The use of heavier gage metal or significant lateral bracing increases the cost of manufacture and production of the metal truss.
The use of tubular pieces, either square or rectangular, as truss web members results in greater strength for any given gauge of metal, however, the tubular must either be cold-rolled and then welded along a seam, or otherwise hot-rolled. Hot-rolled members are more expensive to manufacture than cold-rolled members. Similarly, the additional step of welding cold-rolled tubulars adds cost over cold-rolled tubulars without welding. Additionally, welded tubulars must be sealed after welding to prevent rusting, especially along the welded seam.
Consequently, there is a need for a metal truss and method of manufacture of metal trusses, having web members which are less expensive than a tubular member and which provides adequate strength without the need to use a heavier gauge metal.